1. Field of the Invention
This invention pertains to subsurface borehole underreamers. More particularly, it pertains to underreamers especially suited for use with reverse circulation.
2. Prior Underreamers and Their Problems
A subsurface borehole underreamer is a tool which is used to enlarge a portion of the length of a hole drilled in the earth below a restriction in the hole. Such tools are used in drilling oil, gas, water, mining, and construction holes and wells, and also in the formation of shotholes for blasting. They are also used in reworking previously drilled oil wells. An underreamer has two operative states, a collapsed or closed state in which the tool diameter is sufficiently small to allow it to be moved in the hole past the restriction, and an opened or expanded state in which the diameter of the tool corresponds to the desired greater diameter to which the hole is to be enlarged below the restriction. As the tool is opened, one or more arms, hinged to the tool body and carrying suitable cutters, pivot out from the body to position the cutters for enlargement of the hole as the tool is thereafter operated; such operation includes rotating the tool and lowering it as it is rotated.
Underreamers are now of two general types, the so-called rock-types and drilling types. Rock-type underreamers are used where the entire length of the borehole, at least over the length thereof to be underreamed, has previously been drilled. Rock-type underreamers have large cutters which extend in the body to its center when the tool is closed; in such tools it is not required that a circulating fluid (air or a suitable liquid) flow axially through the tool from end to end. In drilling-type underreamers, on the other hand, it is required that a circulating fluid flow from end to end of the tool when it is opened. Drilling-type underreamers, therefore, use smaller cutters which, when the tool is closed, do not fully extend to the center or axis of the tool, thereby providing room in the tool for the definition of a circulating fluid duct past the retracted position of the cutters. In a drilling-type underreamer, the cutters are located between the exterior of the circulation duct and the exterior of the tool body when the tool is closed. Rock-type underreamers, therefore, enable a hole of given diameter to be enlarged to a greater diameter than do drilling-type underreamers due to the fact that they incorporate larger cutters within the interior of the tool body than a drilling-type underreamer.
A drilling-type underreamer usually is used in conjunction with a drill bit below the underreamer. The underreamer is a lower component of a string of rotary drill pipe, and the drill bit is carried at the lower end of the string. The drill bit forms the hole to be underreamed at the same time that the underreamer enlarges the hole formed by the bit. Circulation of fluid must be provided to the drilling bit to remove cuttings created by the bit as it is operated.
The advantages of existing rock-type underreamers are that they enable the use of the largest possible cutters within the confines of the tool body, and they afford maximum expanded diameter for a given size of the tool body; their disadvantages are that they provide no communication of circulating fluid below the tool, no direct fluid wash is provided to the cutters as they are operated, and it is not possible to use hydraulically or non-hydraulically actuated tools below such underreamers. The advantages of drilling-type underreamers are that they provide fluid communication below the tool, they enable the provision of a fluid wash on the underreamer cutters, and they enable the use of hydraulically or non-hydraulically actuated tools in the drill string below the underreamers; their disadvantages are that, for a given tool size, they can accommodate within the confines of the tool body only smaller cutters, and, therefore, their expanded diameter is limited as compared to rock-type underreamers.